Solar Panel Size: How To Calculate Solar Panel, Battery, and Inverter

Solar panels, solar batteries, charge controller, and inverter must be properly dimensioned in order to provide the user with enough power and energy when required, regardless if the system is used only in emergencies or blackouts, or as an off-the-grid power system.

When calculating the performances of the power inverter, solar battery, and solar panels, some assumptions must be taken regarding the efficiency of solar panels, required energy, average and peak power, and similar. However, one also must be aware that over-dimensioning certain components can increase their price significantly.

Published: April 11, 2022.

How to Choose Power Inverter

A power inverter converts chemical energy stored in the battery into the electric energy of the required voltage and waveform.

The most important features of power inverters are:

- Output Power is the power that the power inverter can provide continuously for a longer period of time. For most homes, RVs, or similar off-the-grid systems, power inverters feature 2000-5000 watts, although even stronger units can be found.

- Surge Power is the power that the power inverter can provide for a very short period of time, which is very important when powering units, tools, and appliances that feature electric motors that draw more current when starting.

- Input Voltage is the voltage of the battery pack that is connected to the power inverter. Most power inverters support the use of 12V batteries, but higher voltages (24V, 36V, 48V, etc.) improve energy efficiency.

- Energy Efficiency of the power inverter depends on the model, but also on the load on the power inverter, and it varies usually between 80 and 95%.

- Output Ports are usually 120 volts AC although some models also feature USB charging ports.

- Wave Form - the best power inverters feature pure sine waveform and are suitable for powering sensitive equipment, including laptops, TVs, and similar electronics. The THD (Total Harmonic Distortions) levels should be below 3%.

Of course, there are other details that may be important in certain situations, but they are highly individual.

So, when choosing the power inverter, choose according to your own needs and requirements.

Note: many solar charge controllers are all-in-1 devices that have a power inverter function already built-in.

Now, it is very important to write down how many watts the power inverter must provide and for how many hours.

For example, we need a power inverter that is able to provide 1000W continuously, for 8 hours during the night - a 2000W pure sine power inverters fit perfectly for such requirement.

That means that our power inverter provides 8 kWh every night of energy and if we assume that its energy efficiency is 85%, that means that the battery (or battery pack) must be able to deliver ~9.5 kWh of energy.

How to Choose Solar Battery

A solar battery or solar battery pack stores collected solar energy and provides power to the power inverter.

Thanks to the advancements in technology, for applications that cycle a lot, lightweight lithium batteries are recommended - these batteries also feature much better energy efficiency than lead-acid batteries, up to 90% vs 50-60%, depending on the charging/discharging conditions.

When looking for a deep-cycle lithium battery, be sure to check:

- Continuous Charging/Discharging Current,

- Surge Charging/Discharging Current,

- supported number of charging/discharging cycles,

- allowed number of batteries in series and/or parallel connections,

- nominal capacity and voltage,

- battery terminals.

Again, there are other details that may be important in certain situations.

The following comparison chart lists some of the most popular 12V lithium batteries with their most important features and specifications:

Note: Amazon affiliate links ("Model" column) open in the new windows, feel free to check them for the most up-to-date offers and prices.

So, if we want to create a battery pack that is able to provide 9.5 kWh of energy every night, that means that we may use 12V 200-400Ah batteries that support connections in parallel and/or series.

For example, if we have a 12V, or 24V, or 48V power inverter, we can connect four Ampere Time 12V 200Ah batteries for a total of ~9.6 kWh:

- 12V: 4 batteries in parallel (1S4P),

- 24V: 2 in series, 2 in parallel (2S2P),

- 48V: 4 batteries in series (4S1P).

If somebody has a 36V only power inverter, it is possible to use six Ampere Time 12V 200Ah batteries:

- 36V: 3 in series, 2 in parallel (3S2P) for a total of ~14.4 kWh,

or to keep things simple, a three Ampere Time 12V 300Ah batteries:

- 36V: 3 in series, 1 in parallel (3S1P) for a total of 10.8 kWh.

Personally, when doing planning of such systems it is highly recommended to use a safety margin of up to or even more than 50-100% - this increases initial costs, but in the long run, the energy/power requirements in most situations can only go up ...

So, if the battery pack must provide ~9.5 kWh of energy every night, we can go for a battery pack that consists of four (4) Ampere Time 12V 200Ah or some other similar 12V 200Ah lithium batteries.

Battery packs, even lithium ones, are not ideal - they also feature energy losses which can range usually around 80-90%. In our example, we will assume that these batteries feature an energy efficiency of around 85% - this is the real-life energy efficiency of lithium batteries which depends on charging/discharging conditions.

So, if we have a battery pack with an energy efficiency of 85% that must deliver ~9.5 kWh, and is able to deliver ~9.6 kWh, that also means that the solar charge controller must be able to provide ~11.3 kWh of energy.

Since the energy efficiency of solar charge controllers also varies in the 80-90% range, if we assume again 85% energy efficiency, that means that the solar panels must provide ~13.3 kWh of energy to the solar charge controller during a single day.

How To Choose Solar Panels

When looking for "the best" solar panels it is good to know their solar efficiency, their physical dimensions and weight, output voltage and currents, and similar - they must be compatible with the solar charge controller/power inverter.

But, for this article, it is important to calculate the number and size of solar panels. And again, we must assume a few things first, including:

- daylight hours differ depending on the location, season, and similar, but we will assume 12h of usable daylight.

- solar panels' energy efficiency depends on the sun's position, solar panels' angle and orientation in general, weather conditions, etc. We will assume 50% solar panels energy efficiency which may sound low, but if one doesn't have solar panels with a sun tracking and solar panels automatic orientation system (which can be pricy, but...), it is really hard to expect higher energy efficiency.

So, if the solar panels must collect ~13.3 kWh during 12h of daylight, that means on average ~1.11 kWh per hour, or 1.11 kW effectively, or ~2.22 kW of installed solar panels.

Long Story Short: If we want our power inverter to provide 1000W for 8 hours every night, the system described in this article requires ~23 solar panels with a nominal power of 100W each, or 11 solar panels with a nominal power of 200W each, etc.

Personally, systems like these are not simple nor cheap, and they must be designed and installed by certified professionals according to the individual needs and requirements, but also local laws, climate, and other conditions.

Whatever You do, it is your own responsibility ... stay safe ...